An Cosantóir

An Cosantóir June 2019 www.dfmagazine.ie 26 | O R D N A N C E CO R P S A R T I C L E S ER I E S REPORT & PHOTOS BY CAPTAIN ORDNANCE SCHOOL, DFTC The Steyr Mod 14 rifle fires 5.56 x 45 mm ammunition. The meaning of 5.56 x 45 mm is not as obvious as it may seem. 45 mm refers to the length of the cartridge case – not the bullet. 5.56 mm refers, not to the ammunition, but to the barrel through which it's fired! You may recall that the sepa- ration between a rifle barrel's 'lands' defines the calibre – in this case 5.56 mm. Figure 1: 5.56 x 45 mm "SS109" Ball Ammunition. For a bullet to be fired at speed, it needs to 'obturate' in the barrel. Simply put, propel- lant gases must not escape the barrel before the bullet. Remember that the gases are what push the bullet down the barrel and towards the target. (Mortar ammunition has a plastic 'o-ring' to achieve this.) Small-arms achieve obturation by squashing the bullet into the barrel. Think about this! Before firing, the bullet's wider than the rifling (at approximately 5.69 mm)! During fir- ing, the projectile is squashed, becoming slightly longer and slimmer as it travels down the barrel. Figure 2: Side profile of SS109 Ball Ammunition. Note the cannelure used to crimp the cartridge case to the Bullet. Standard "ball 1 " ammuni- tion has a lead body with a copper outer lining (known a jacket). The jacket covers all but the base of the bullet – as seen in Figure 3. SS109 Bullet Base and Pre-Fire Diameter. The un-jacketed base is very im- portant since it allows for the bullet to change shape when engaging in the rifling. This is also where the pyrotechnic composition is placed for tracer ammunition. Figure 3: SS109 Bullet Base and Pre-Fire Diameter. Note the exposed core. The problem with ball am- munition is that it's hard to see the 'fall of shot'. Hence, we modify the ball bullet to create tracer ammunition. So how does tracer work? Tracer ammunition uses a pyrotech- nic mix in the base of the bullet (where there is no jacket) to emit light. In fact, two different pyrotechnics are used. The first pyrotechnic composition burns 'dark' to protect the user's eyesight. It then ignites the primary tracer mix mid-flight. Therefore, tracer light is best viewed from directly behind it. So now that you understand how tracer ammunition works, let's discuss the ballistics of tracer. What does 'bal- listics' even mean? It's a broad term, which encompasses everything experienced by ammunition from being fired, to when it impacts a target. It's broken into four groups: inter- nal; intermediate; external and terminal. Soldiers are inter- ested in ballistics – whether they realise it or not! The range is described by internal, intermediate and external ballistics. The effects on target are described by terminal ballistics. So then, what are the ballistics of tracer? Preferably, ball and tracer ammunition should perform identically. Tracer ammunition is longer – to fit the pyro- technic composition. This alters the ballistics. Recall that the pyrotechnic mix burns away during flight. Hence, the mass/ weight of tracer bullets change! This further complicates the ballistics. Engineers design tracer ammunition to ballistically- match the flight of ball ammunition to a certain point. After this point the trajectories diverge. Also, once the pyrotechnic mix has burned-out, the fall-of-shot is difficult to see. Engineers try to balance the point of tracer-burnout with that of ballistic-matching. Soldiers are interested in the ballis- tics of tracer, up until it deviates from the trajectory of ball am- munition. Since this can't be seen, the point of tracer-burnout is used as the planning range. For those being fired at however, ball and tracer ammunition are indistinguishable! Learn more: Ammunition Examiners are responsible for the safe-storage, inspection and destruction of all 'natures' of service-ammunition. Armourers and Armament Artificers are responsible for the inspection and repair of all firearms. Ordnance Corps' technicians have a wealth of knowledge and are always happy discuss their trades! Further reading: Bailey, A. and Murray, S. G., (1989), Explosives, Propellants & Pyrotechnics, Volume 2, Shrivenham: Brassey's. Courtney-Green, P. R., (1991), Ammunition for the Land Battle, Volume 4, Shrivenham: Brassey's. Ordnance Corps (2016), Ballistics Precis, DFTC, Defence Forces. 1 Named after the Minie Ball projectile. How Your Equipment Works: The Ballistics of Tracer

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